FERTILISATION
The percentage of successfully fertilised eggs steeply increased with
sperm concentration but dropped significantly beyond 1 ×
106 (Fig. 1a). Beyond this concentration,
~60 to 80% of eggs were abnormally fertilised (Fig.
1b). Therefore, had we simply counted fertilised vs unfertilised eggs,
without making a distinction between normally vs abnormally fertilised
eggs, we would have seen a continuous increase in proportion of
fertilised eggs (Fig. 1c) while in fact many of the fertilised eggs at
the highest sperm concentration (1 × 108) were
non-viable.
In the first Bayesian model, we analysed the proportion of normally
fertilised eggs after excluding the highest sperm concentration (1 ×
108). There was a significant effect of population of
origin, sperm concentration (both linear and quadratic terms), female
density, and a significant interaction between female density and sperm
concentration (Table 2). Females from low density areas had lower
fertilisation success across all sperm concentrations (Fig. 1a), but the
significant interaction indicated that this effect becomes less
important as sperm concentration increases (Table 2). We detected no
effect of male density on fertilisation success (Table 2). In the second
model, in which we analysed the proportion of abnormally fertilised eggs
at the highest sperm concentration (1 × 108), we found
no significant effect of population, body condition, gamete age, female
density, male density (or their interaction) on the proportion of
abnormally fertilised eggs (Table 3).
Finally, our third Bayesian model revealed some differences in the
variance estimates across sperm concentrations. The variance estimates
associated with intrinsic male effects (V male)
were low across all sperm concentrations (range: <0.01 to
0.01; Table 4). By contrast, effects attributable to females
(V female) were relatively high (range: 0.62 to
4.96; Table 4). The “male × female” interaction effects
(V male × female) were relatively low at low sperm
concentration (Fig. 2), but increased at higher sperm concentration
(Table 4, Fig. 2).